CRISES IN ORGANIC EVOLUTION. 457 



constitutes the ideal genealogical tree of the animal kingdom; their advents create 

 the real subdivisions, and mark the starting points of divergent evolution for 

 phyla, classes, and for the innumerable smaller branches. The gaps in a natural 

 system of classification therefore do not always mark the periods of lost records 

 or the areas of densest ignorance. 



Hence organic readjustments, according to their import, mark the periods of 

 relatively rapid phyletic changes that are of value in classification. Let us con- 

 sider some of those organic changes that are recognizably correlated with differ- 

 ences in bodily form, and that have been instrumental in fixing or guiding the 

 course of evolution in the arthropod-vertebrate stock. 



A. The Evolution of Metamerism and Bilateral Symmetry. It has been 

 shown that a local exaggeration of radial growth results in apical growth, and that 

 under the existing conditions apical growth inevitably creates bilateral symmetry 

 and a double series of graded unlike conditions and structures, one series extend- 

 ing in a cephalo-caudal direction giving rise to metamerism, the other in a neuro- 

 haemal direction, giving rise to the graded series of conditions and organs on the 

 right and left half of each metamere. (See Fig. 157.) These conditions are prob- 

 ably resident in all coherent, organic growth, for we see essentially the same 

 conditions repeated wherever apical growth prevails, whether in plants or animals. 



It was, no doubt, some local exaggeration of radial growth in a ccelenterate 

 ancestor that initiated the craniate stock. The craniates began as minute ani- 

 mals (trocosphere stage) consisting of a relatively large head that was built on a 

 radiate plan, and that represented the ancestral ccelenterate body. From this 

 primitive head, a local outgrowth arose that gave rise to a new body. The latter 

 gradually increased in volume, and as a result of its characteristic mode of apical 

 growth, bilateral symmetry, metamerism, and the difference between the neural 

 and haemal surfaces, became gradually and permanently established. 



The old body, or primitive head, decreased relatively in volume and was 

 finally represented almost solely by its nervous elements, the primitive forebrain, 

 and by the oral opening into the alimentary canal. The new trunk increased in 

 volume by the spasmodic production of new groups of metameres, alternating 

 with prolonged periods of phylogenetic inactivity. Each new group of metameres 

 always differed from the preceding ones, and after a varying period of greater 

 or less organic independence, was incorporated with them as a subordinate part 

 of an increasingly complex head. Even in its most elaborate condition, as seen 

 in the higher vertebrates, the head still shows distinct traces of the successive gen- 

 erations of metameres of which it is composed. 



Each notable increase in the size of the body due to the addition of a new 

 group of metameres, disturbed the pre-existing organic equilibrium, bringing 

 about the shifting of the organs of locomotion, excretion, circulation, and digestion, 

 to regions farther and farther back in the body. Outgrowths of nerve fibers from 

 the old sensory and nervous centers then established coordinating relations with 

 them. 



